Polycrystalline diamond compacts (“PDC”) have been used in industrial applications, including rock drilling applications and metal machining applications. Such compacts have demonstrated advantages over some other types of cutting elements, such as better wear resistance and impact resistance. The PDC can be formed by sintering individual diamond particles together under the high pressure and high temperature (“HPHT”) conditions referred to as the “diamond stable region,” which is typically above forty kilobars and between 1,200 degrees Celsius and 2,000 degrees Celsius, in the presence of a catalyst/solvent which promotes diamond-diamond bonding. Some examples of catalyst/solvents for sintered diamond compacts are cobalt, nickel, iron, and other Group VIII metals. PDCs usually have a diamond content greater than seventy percent by volume, with about eighty percent to about ninety-five percent being typical. An unbacked PDC can be mechanically bonded to a tool (not shown), according to one example. Alternatively, the PDC can be bonded to a substrate, thereby forming a PDC cutter, which is typically insertable within a downhole tool (not shown), such as a drill bit or a reamer.
FIG. 1 shows a cross-sectional view of a PDC cutter 100 having a polycrystalline diamond (“PCD”) cutting table 110, or compact, in accordance with the prior art. Although a PCD cutting table 110 is described in the exemplary embodiment, other types of cutting tables, including cubic boron nitride (“CBN”) compacts, are used in alternative types of cutters. Referring to FIG. 1, the PDC cutter 100 typically includes the PCD cutting table 110 and a substrate 150 that is coupled to the PCD cutting table 110. The PCD cutting table 110 is about one hundred thousandths of an inch (2.5 millimeters) thick in the thickest portions of the PCD cutting table 110; however, this thickness can vary depending upon the application.
The substrate 150 includes a top surface 152, a bottom surface 154, and a substrate outer wall 156 that extends from the circumference of the top surface 152 to the circumference of the bottom surface 154. The top surface 152 is non-planar, but can be substantially planar in certain embodiments. The non-planar top surface 152 includes one or more columns 153 that extend substantially upwards in a vertical direction with respect to the bottom surface 154. However, in other embodiments, the non-planar top surface 152 includes bump and valleys or any other protrusions and/or indentations thereby making the top surface 152 non-planar. The PCD cutting table 110 includes a cutting surface 112, an opposing surface 114, and a PCD cutting table outer wall 116 that extends from the circumference of the cutting surface 112 to the circumference of the opposing surface 114. According to some exemplary embodiments, a bevel (not shown) is formed around at least the circumference of the PCD cutting table 110. The opposing surface 114 is non-planar and complementary to the top surface 152, but can be substantially planar in certain embodiments. The opposing surface 114 of the PCD cutting table 110 is coupled to the top surface 152 of the substrate 150 and surrounds the columns 153, or other protrusion types. Typically, the PCD cutting table 110 is coupled to the substrate 150 using a HPHT press. However, other methods known to people having ordinary skill in the art can be used to couple the PCD cutting table 110 to the substrate 150. In one embodiment, upon coupling the PCD cutting table 110 to the substrate 150, the cutting surface 112 of the PCD cutting table 110 is substantially parallel to the bottom surface 154 of the substrate 150. Additionally, the PDC cutter 100 has been illustrated as having a right circular cylindrical shape; however, the PDC cutter 100 is shaped into other geometric or non-geometric shapes in other embodiments.
According to one example, the PDC cutter 100 is formed by independently forming the PCD cutting table 110 and the substrate 150, and thereafter bonding the PCD cutting table 110 to the substrate 150. Alternatively, the substrate 150 is initially formed and the PCD cutting table 110 is then formed on the top surface 152 of the substrate 150 by placing polycrystalline diamond powder onto the top surface 152, including around the columns 153, and subjecting the polycrystalline diamond powder and the substrate 150 to a high temperature and high pressure process. Although two methods of forming the PDC cutter 100 have been briefly mentioned, other methods known to people having ordinary skill in the art can be used.
According to one example, the PCD cutting table 110 is bonded to the substrate 150, formed from a material such as cemented tungsten carbide, by subjecting a layer of diamond powder and a mixture of tungsten carbide and cobalt powders to HPHT conditions. The cobalt diffuses into the diamond powder during processing and therefore acts as both a catalyst/solvent for the sintering of the diamond powder to form diamond-diamond bonds and as a binder for the tungsten carbide. Voids are formed between the carbon-carbon bonds of the diamond. Strong bonds are formed between the PCD cutting table 110 and the cemented tungsten carbide substrate 150. The diffusion of cobalt into the diamond powder results in cobalt being deposited within the voids formed within the PCD cutting table 110. Although some materials, such as tungsten carbide and cobalt, have been provided as examples, other materials known to people having ordinary skill in the art can be used to form the substrate 150, the PCD cutting table 110, and form bonds between the substrate 150 and the PCD cutting table 110.
Since the cobalt, or catalyst material, is deposited within the voids formed within the PCD cutting table 110 and cobalt has a much higher thermal expansion rate than diamond, the PCD cutting table 110 becomes thermally degraded at temperatures above about 750 degrees Celsius and its cutting efficiency deteriorates significantly. Hence, typical leaching processes, which are known to people having ordinary skill in the art, have been used to react the deposited catalyst material, thereby removing the catalyst material from the voids.
All typical leaching processes involve the presence of an acid solution (not shown) which reacts with the catalyst material that is deposited within the voids of the PCD cutting table 110. According to one example of a typical leaching process, the PDC cutter is placed within an acid solution (not shown) such that at least a portion of the PCD cutting table 110 is submerged within the acid solution. The acid solution reacts with the catalyst material along the outer surfaces of the PCD cutting table 110. The acid solution slowly moves inwardly within the interior of the PCD cutting table 110 and continues to react with the catalyst material. However, as the acid solution moves further inwards, the reaction byproducts become increasingly more difficult to remove; and hence, the rate of leaching slows down considerably. For this reason, a tradeoff occurs between leaching process duration, wherein costs increase as the leaching duration increases, and catalyst removal depth. Typically, the leaching process is performed to allow a catalyst removal depth of about two millimeters; however, this depth can be increased or decreased depending upon the application of the PCD cutting table 110 and/or the cost constraints.
FIG. 2A shows a cross-sectional view of the PDC cutter 100 of FIG. 1 having a wear flat 210 in the PCD cutting table 110 in accordance with the prior art. FIG. 2B shows a side elevational view of the PDC cutter 100 of FIG. 2A in accordance with the prior art. Referring to FIGS. 2A and 2B, the wear flat 210 develops on a portion of the circumference of the PCD cutting table 110 when the portion of the PCD cutting table 110 is worn out by the interaction occurring between the PCD cutting table 110 and the rock formation. When the wear flat 210 is formed, eventually at least a portion of one or more columns 153 of the substrate 150 becomes exposed for cutting the rock formation. An interface 220 is formed within the wear flat 210 where the PCD cutting table 110 meets with the column 153. Since the PCD cutting table 110 is formed substantially from diamond or other known material, and the columns 153 are formed substantially from tungsten carbide or other known material, the interaction occurring between the PCD cutting table 110 and the rock formation and between the column 153 and the rock formation causes the column 153 to wear faster than the surrounding PCD cutting table 110. Thus, the exposed portion of the column 153 becomes slightly recessed into the wear flat 210 when compared to the exposed portion of the PCD cutting table 110 within the wear flat 210. Hence, the PDC cutter 100 provides for a claw cutting action occurring between the interfaces 220 and the rock formation in addition to the cutting actions performed individually by the PCD cutting table 110 and the columns 153. This cutting action provides for improved cutting and a higher rate of penetration (“ROP”). Typically, the worn PDC cutter 110 or the entire downhole tool that includes several of these worn PDC cutters 110 are replaced once the columns 153 become exposed or soon thereafter; thereby having the benefits of the claw cutting action not being fully realized.
The drawings illustrate only exemplary embodiments of the invention and are therefore not to be considered limiting of its scope, as the invention may admit to other equally effective embodiments.